Chronic substance use (SU) and licit and illicit substance use disorders (SUDs) are debilitating and contribute substantially to global morbidity and mortality. Personality and personality disorders strongly influence risk of SU and SUDs, but we know little about the etiologic pathways involved. This project utilizes a unique resource to address a number of novel research questions concerning SU and SUDs. We will use state-of-the-art genetic epidemiological methods to identify causal and non-causal pathways from normal, abnormal and pathological personality to licit and illicit SU and SUDs using the longitudinal, phenotypically rich, and genetically informative population-based Norwegian Twin Registry (NTR). We will link the NTR data to national, population-based and clinical registries, using Norway's unique personal identification number to study the risk factors, causes and disease mechanism as well as consequences of licit and illicit SU and SUDs. This will enable validation and identification of unreported SUD cases. The registry data will also provide objective information on demographics, socioeconomic status and psychosocial dysfunction as measured by sick leave, disability, and illness thereby enabling us to determine the causal and non-causal pathways from SU and SUD to psychosocial dysfunction. Our project has 6 scientific and one operational aim. We will use cross-sectional and longitudinal data to estimate the nature of the phenotypic associations between disinhibitory normal, abnormal and pathological personality types, SU and SUD, and correlated Axis I disorders before decomposing the associations into genetic and environmental pathways. Using complementary approaches we will model direction of causation between our key variables, while testing meditational models to clarify the role of SU as a gateway to SUDs and correlated externalizing spectrum disorders. Using genetically informative longitudinal models we will explore the causal relationship between alcohol use disorders and major depression. We will also merge NTR twin data with data from Statistics Norway to examine the phenotypic and genetic pathways from SU and SUDs to objective psychosocial dysfunction as indexed by nationwide sick leave, disability and illness. Our operational aim is to ascertain a new, fourth wave of questionnaire data on lifetime SU and SUDs from the NTR twins. This will provide statistical power to test competing longitudinal models. The modelling expertise of the collaborative group will ensure optimal exploitation of these resources. This application is submitted under a special funding mechanism Norwegian Collaborative Projects with Research Groups in the United States established and cofounded by the NIH and the Research Council of Norway. We propose to study how genetic and environmental risk factors in normal, abnormal and pathological personality impact and predict the risk of SU and SUDs by linking new and existing data from the NTR to national, population-based demographic and clinical registries. The same data will provide objective information on how SU and SUDs are related to sick leave, illness and disability.